The present invention relates to III-V compound semiconductor devices represented by semiconductor laser devices used as optical information system light sources for CD, MD and DVD players or computer information storage devices, and to a manufacturing method thereof. The present invention relates, in particular, to a structure for achieving a low threshold current operation, to a semiconductor device excellent in device characteristics, yield and reliability with improved controllability of impurities included in semiconductor layers, and to a manufacturing method thereof.
In recent years, there has been a growing demand for semiconductor laser devices that are compound semiconductor devices to be used for the pickups of CD and MD. Semiconductor laser devices that have little characteristic variations and excellent reliability have been demanded. Also, it is anticipated that the demand for semiconductor laser devices will be still more increasing in future for the production of the computer information storage devices such as CD-ROM, CD-R, CD-RW, and digital video discs (DVD).
When producing a III-V compound semiconductor device represented by such a semiconductor laser device, a stacked structure of a plurality of semiconductor layers is formed on a semiconductor substrate. By adding a specified impurity to each semiconductor layer, the electric conduction type or the electric conductivity of each layer is controlled to consequently obtain a device of specified semiconductor characteristics. To achieve uniform device characteristics of the semiconductor lasers and improvement in yield of products, it is very important to control the electric conduction type or the electric conductivity of each layer of the semiconductor device to be in conformity with designed values.
As a method of forming III-V compound semiconductor thin films in a stacked manner, the MOCVD (metal-organic chemical vapor deposition) method and the MBE (molecular beam epitaxy) method can be mentioned. When growing a film by using any of these methods, a group IV element such as silicon (Si) and a group VI element such as selenium (Se) are used as impurities for obtaining an n-type electric conduction type layer. The group IV element becomes a donor impurity by replacing a group III element of aluminum (Al), gallium (Ga), or indium (In). The group VI element becomes a donor impurity by replacing a group V element of arsenic (As) or phosphorus (P). On the other hand, as an impurity for obtaining a p-type electric conduction layer, a group II element such as zinc (Zn), beryllium (Be), or magnesium (Mg) is employed. The group II element becomes an acceptor impurity by replacing a group III element of Al or Ga.
Among semiconductor laser device structures, what we call a self-alignment structure and what we call a ridged structure are well known. FIGS. 4A, 4B and 4C show an example of a semiconductor laser device of the self-alignment structure. The fabricating process of this semiconductor laser device will be described below.
In the first process step shown in FIG. 4A, first, an n-type GaAs buffer layer 12 (layer thickness: 0.5 μm), an n-type AlxGa1-xAs first cladding layer 13 (x=0.5, layer thickness: 1.0 μm), a non-doped AlxGa1-xAs active layer 14 (x=0.14, layer thickness: 0.085 μm), a p-type AlxGa1-xAs second cladding layer 15 (x=0.5, layer thickness: 0.35 μm) and an n-type GaAs current block layer 16 (layer thickness: 0.6 μm) are successively grown on an n-type GaAs substrate 10 by the MOCVD method. In this stage, Se is employed as the n-type impurity, while Zn is employed as the p-type impurity. Next, in the second process step shown in FIG. 4B, an etching mask 40 is formed by a method such as photolithography. Thereafter, the n-type GaAs current block layer 16 is removed in a stripe-like and groove-like shape with a width of 3.5 to 4.0 μm, forming a removed portion 20.
Subsequently, in the third process step shown in FIG. 4C, a p-type AlxGa1-xAs third cladding layer 17 (x=0.5, layer thickness: 1.0 μm) and a p-type GaAs cap layer 18 (layer thickness: 3 to 50 μm) are grown on the n-type GaAs current block layer 16 including the removed portion 20 by the MOCVD method or the LPE method. In this case, the layer thickness of the p-type GaAs cap layer 18 should be determined as the occasion demands depending on where the final light emitting point of the semiconductor laser device is to be positioned relative to the chip thickness. Zn or Mg is employed then as the p-type impurity. By the aforementioned fabricating method, the semiconductor laser device of the self-alignment structure is obtained.
The molar ratio of the group V element to the group III element (V/III ratio) when forming a laminate by the MOCVD method in the first process step has conventionally been set to 20 to 150 at a growth temperature of 600° C. to 800° C. If the ratio is set to a value of 20 or lower, then there occurs a phenomenon of roughened growth surface. On the other hand, it has been reported that if the growth temperature is set to 450° C. to 600° C., then no roughness occurs on the crystal surface even when the V/III molar ratio is reduced to 0.3 to 2.5, and that the intake of carbon C to the grown thin film is increased so that a p-type hole density of 1×1018 cm−3 to 1×1020 cm−3 by the carbon C of GaAs and AlGaAs is obtained (JP-B2-2885435).
In a practically used semiconductor laser device of the structure shown in FIG. 4C, in the first process step for forming at least the n-type first cladding layer 13, the active layer 14, the second cladding layer 15 and the n-type current block layer 16 on the n-type GaAs substrate, the n-type first cladding layer 13 and the n-type current block layer 16 are doped with an impurity of Se, and the p-type second cladding layer 15 is doped with an impurity of Zn However, in the structure after the completion of the first process step, the impurity elements move or migrate between the layers by diffusion or the interaction of the impurity atoms during the fabricating process, which results in an impurity profile different from a designed impurity profile. FIG. 3A shows the designed impurity concentration profile, in which, of course, the n-type first cladding layer 13 and the n-type current block layer 16 are designed to be doped with the n-type impurity of Se, and the p-type second cladding layer 15 is designed to be doped with the p-type impurity of Zn, each with a steep doping slope. FIG. 3B shows an actual impurity concentration profile. As obvious from this figure, the impurity of Zn in the p-type second cladding layer 15 diffuses into the layers other than the p-type second cladding layer 15 during the growth of the n-type current block layer 16 in the first process step, as a consequence of which the doping control of the p-type second cladding layer 15 becomes unstable.
Further, in the third process step after the formation of the stripe removed portion 20 in the n-type current block layer 16 in the second process step, due to a thermal history during the process for growing the p-type third cladding layer 17 and the p-type GaAs cap layer 18 at the removed portion of the current block layer 16 and the non-removed portion of the current block layer, the impurity of Zn in the p-type second cladding layer 15 increasingly diffuses into the other layers and, in certain circumstances, the impurity of Se of the n-type first cladding layer 13 and the n-type current block layer 16 diffuses into the p-type second cladding layer 15. The diffusion of n-type impurity surpasses the concentration of the p-type impurity of Zn of the p-AlGaAs cladding layer 15, consequently causing the inversion of the p-type second cladding layer 15 into the n-type. This inversion into the n-type, which occurs either on the entire surface of the p-type second cladding layer 15 or in the portion that faces the non-removed portion of the n-type current block layer 16, disables the local current injection for obtaining the laser oscillation of the semiconductor laser device, causing a defective product.
Also, in the ridged-structure semiconductor laser device as well, Zn has been employed as the impurity added to the p-type cladding layer to be formed on an active layer constructed of a quantum well layer. Therefore, similar to the semiconductor laser device of the self-alignment structure, Zn disadvantageously diffuses into the active layer during the fabricating process. This causes the disorder of the entire quantum well active layer, eventually changing the oscillation wavelength. The diffusion otherwise causes the degradation in crystallinity of the quantum well active layer, disadvantageously increasing the threshold current and the operating current. This has resulted in degradation in laser characteristics and an increase in characteristic variations.